Mattress foundations or “box springs” are commonly used as the support structure or system underneath a mattress, to provide the mattress with a mounting surface which is firm but which also has some degree of flexibility for responding to heavier loads. Mattress foundations have traditionally been constructed with a frame, such as a wooden frame, which supports multiple spring elements or modules which in turn support a grid or platform for the mattress. The foundation structure is covered by upholstery and other material covering and some padding. The shape of the mattress foundation is compatible with the matching mattress, i.e. planar top and bottom sides, vertical side walls and generally rounded corners at the intersections of the vertical sides. The rounded corners of the foundation present a design challenge due the general absence of structural support of the grid by the spring members at the corners. When the corners are left essentially unsupported, this leads to sagging of the foundation and the mattress in those areas. Also the absence of internal structure at the corner areas of the foundation leaves the covering or upholstery without any backing, which can lead to overstretching or wrinkling of the material and an unattractive finished appearance.